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SEAWATER.TXT
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1991-02-07
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Stainless steels are susceptible to crevice attack in chloride bearing
waters. Their behavior has been studied by a number of investigators.
There is considerable variation in the percentage of apparently identical
sites where attack occurs, when it occurs. It is useful to describe results
in terms of the percentage of apparently identical sites where attack occurs
at a given chloride concentration. Very tight stationary crevices increase
the likelihood of attack. Rough surfaces, sheared edges, scratches and
similar imperfections also tend to increase the incidence of attack.
Crevice attack also occurs under deposits and under biofouling growths
attached to the surface.
Relative resistance can be described by the chloride concentration below
which there is little likelihood of crevice attack occurring. The ability
of chlorides to concentrate in some crevices means that occasional attack
may occur at lower concentrations than shown in the following table.
Nevertheless, the table provides useful guidelines.
TABLE
Guidelines for relative resistance of stainless steels
to crevice attack in natural waters
Stainless steel Chloride concentration below which
crevice attack is rare
430 & series 400 Crevice attack occurs in fresh waters
304 100-200 ppm
316 1000 ppm
The 4 1/2% Mo and duplex stainless steels are more resistant than type 316,
but suffer varying degrees of crevice attack in brackish water and seawater.
The 6% Mo stainless steels have excellent resistance to crevice attack in
seawater.
For further information: NiDI 11 003 "Guidelines for Selection of Stainless
Steels for Marine Environments, Natural Waters and Brines"